Coolant pumps in internal combustion engines serve to control the flow of the delivered coolant to prevent the internal combustion engine from overheating. These pumps are in most cases driven via a belt or a chain drive so that the coolant pump impeller is driven at the speed of the crankshaft or at a fixed ratio to the speed of the crankshaft.
In modern internal combustion engines, the delivered coolant flow must be matched with the coolant demand of the internal combustion engine or the motor vehicle. The cold running phase of the engine should in particular be reduced to prevent increased pollutant emissions and to reduce fuel consumption. This is realized, inter alia, by restricting or completely switching off the coolant flow during this phase.
Various pump designs for controlling coolant flow rate are known. Besides electrically driven coolant pumps, pumps are known which can be coupled to or decoupled from their drive units via couplings, in particular hydrodynamic couplings. A particularly inexpensive and simple manner of controlling the delivered coolant flow is the use of an axially movable control slide which is pushed across the coolant pump impeller so that, for reducing the coolant flow, the pump does not deliver into the surrounding delivery duct but against the closed slide.
The control of this slide is also performed in different ways. Besides a purely electric adjustment, a hydraulic adjustment of the slides has in particular proved successful. A hydraulic adjustment is in most cases carried out via an annular piston chamber which is filled with a hydraulic fluid and whose piston is connected to the slide so that, during filling of the chamber, the slide is moved across the impeller. The slide is returned by opening the piston chamber towards an outlet, in most cases via a magnetic valve as well as by a spring action providing the force for returning the slide.
For the coolant flow required for moving the slide not to be supplied via additional delivery units, such as additional piston/cylinder units, or for other hydraulic fluids not to be compressed for operating purposes, mechanically controllable coolant pumps are known on whose drive shaft a second delivery wheel is arranged via which the pressure for adjusting the slide is provided. These pumps are designed, for example, as side channel pumps or as servo pumps.
A coolant pump having a side channel pump acting as a secondary pump is described in DE 10 2012 207 387 A1. In this pump, via a 3/2-way valve, in a first position, a discharge side of the secondary pump is closed and a suction side of the pump is connected to the coolant circuit and the slide, and in a second position, the discharge side is connected to the slide and the suction side is connected to the coolant circuit. A spring is used to return the slide, which spring may be omitted when the pump is to be reset by the negative pressure produced at the suction connection.
It is, however, problematic that a sufficient coolant pressure initially does not exist when starting the internal combustion engine, via which the control slide is rapidly moved into its position for closing the duct and thus stopping a coolant flow. A rapid control of the coolant flow is thus not possible directly after the start, in particular at an idle speed, so that the heating times cannot be considerably reduced as in the case of an immediate switch-off by moving the control slide into the annular gap.
For vehicles having an automatic start-stop system, several documents therefore suggest a solution wherein, in addition to a mechanically driven pump, an electric pump is arranged in the coolant circuit to maintain the delivery of the coolant at high coolant temperatures even at low speeds. Such an arrangement is described, for example, in WO 2012/119622 A2. In the therein described cooling system, the control slide is to be moved into its position for closing the duct to prevent an undesired cooling during the start. This is, however, only possible in the case of electrically operated actuators since a sufficient hydraulic pressure to move the control slide is normally not provided at idle speed.